ultrafast ultrasound

B-mode vs ultrafast

B mode:

• image is the sum of A lines

• groups of elements are fired to produce a focused beam and the rest of the elements capture the returning echoes

Ultrafast:

• all elements are fired → produces an unfocused plane wave parallel to the probe

• backscattered wave is measured by all elements at once

• algorithm is sued to reconstruct all the A line at once

how does ultrafast US work?

1. measure the backscattered scan line using the whole array

2. find the amplitude of each constituent back scattered plane wave

3. relate the amplitudes to plane wave we transmitted

4. relationship tells use the scattering distribution

how is ultrafast ultrasound modelled in maths

incident wave is modelled as a plane wave with x (lateral) and z(depth) dimensions:

$P(x,z,t)=\int_0^{T}\hat{p}\left(K_{x},K\!_{z},w\right)e^{i\left(K_{x}x+K_{z}z\right)}\,dx$ (sum of incident wave amplitudes and angular frequencies)

amplitude of the received signal is $\hat{p}*\hat{s}$ (multiplying the fourier domains → same as subtracting scattered K from incident K)

• since $\hat{p}$ is known you can divide the fourier by it to isolate the tissue’s fourier spectrum

spatial frequency fourier spectrum ($\hat{s}$) of the backscattered wave is inverse fourier transformed to find the scatterer distribution:

$$S(x,z,)=\int_{}^{}\hat{S}\left(K_{x},K\!_{z},\right)e^{i\left(K_{x}x+K_{z}z\right)}\,dx$$

advantages of ultrafast

1. image formed from 1 transmit pulse → faster frame rate

disadvantages of UFUS → how is it mitigated

1. no transmit focusing → backscattered amplitudes are low -~> low signal to noise ratio → grainy image

2. poor lateral resolution

mitigated using coherent-angle or incoherent-angle compounding:

• US plane wave is sent at different transmit angles

• frames are averaged

• speckle varies and is therefore removed

reduces frame rate

what is ultrafast doppler?

instead of scanning multiple a line, a single horizontal scan line allows you to take more doppler samples in the same amount of time

• greatly reduces motion artefacts in power doppler as images can now be taken at 80Hz instead of 20Hz

what is functional US

• Power Doppler signal-to-noise ~50x higher than conventional systems, so finer details can be obtained

• Due to:

  • Ultrafast systems taking massive temporal averaging (coherent compounding), several thousands of frames are taken per second

  • Power doppler integrates the signal energy

  • Multiple plane wave angle compounding reduces noise and clutter

  • Short pulse repetition intervals -> strong sensitivity to slow flow

localisation microscopy

1. contrast agent is injected → usually microbubbles

2. if you can image a single microbubble and know it is just one → positional accuracy can be determined more accurately

3. allows us to replace the blurry image with a single sharp point

used in vascular imaging to image very small vessels

what are hte 2 ways you can perform localisation microscopy

1. low microbubble conc and low frame rate→ individual microbubble can be seen on the image

2. high microbubble conc and frame rate (500fps) → compare images to see the absence of microbubbles that have disintegrated → easier to use as no need to dilute bubbles